This invention is directed to methods and apparatus for removing gelled drilling fluid and filter cake from well bores, and more particularly to methods and apparatus for removing gelled drilling fluid and filter cake prior to and during the removal of the drill string after the drilling of the well bore has begun. The invention is further directed to removing gelled mud, filter cake and other debris from equipment used during the drilling of offshore wells such as, but not limited to, subsea assemblies and riser pipes.
In the construction of oil and gas wells, a well bore is drilled into one or more subterranean formations or zones containing oil and/or gas to be produced. The well bore is typically drilled utilizing a drilling rig which has a rotary table on its floor to rotate the drill string or other pipe during drilling and other operations. The drilling rig may also have a top drive mechanism for rotating the drill string which is integral with the traveling block of the rig in addition to or instead of a rotary table. In addition, the well bore can be drilled using a downhole drilling mechanism.
During the drilling of the well bore, drilling fluid, which is also referred to as drilling mud, is circulated through the well bore by pumping it down the drill string through the drill bit. When the fluid exits the drill bit, it flows upwardly back to the surface through the annulus between the walls, or sides of the well bore and the drill string. A variety of different types of drilling fluid exist, and the circulation of such fluid functions to, among other things, lubricate the drill bit, remove cuttings from the well bore as they are produced, exert hydrostatic pressure on pressurized fluid contained in formations and seal off the walls of the well bore so that the fluid is not lost into the permeable subterranean zones penetrated by the well bore.
As the drilling fluid circulates through the well bore, it deposits a filter cake of solids from the drilling fluid, along with gelled drilling fluid over the walls of the well bore. In most instances, after the well bore has been drilled to a desired depth, the drill string is removed and a string of casing is run into the well bore. After the casing string is run in the well bore, cementing operations are usually performed. That is, the pipe is cemented or bonded in the well bore by placing a cementitious slurry, or other settable fluid, in the annulus between the side of the well bore and the casing string.
The cementitious slurry sets into a hard impermeable mass and will bond the casing string to the well bore. Cementing seals the annulus and prevents fluid communication between subterranean zones or to the surface by way of the annulus. In order for the cementing to be successful, the gelled and dehydrated drilling fluid and filter cake deposited on the walls of the well bore must be removed. If an appreciable amount of drilling fluid and filter cake remain on the walls of the well bore, the cementitious fluid will not properly bond thereto and fluid leakage through the annulus and other major problems will result.
Various methods have been used to attempt to remove gelled drilling fluid and filter cake from the walls of a well bore. One method of removing such debris has been to simply increase the velocity of fluid circulated through the drill string and the drill bit after the well bore has been drilled to create a more turbulent flow up the annulus, thereby removing some gelled fluid and filter cake from the well bore walls. Fluids may also be pumped through the casing string at high flow rates as the casing string is lowered into the well to create turbulent flow in the well bore. The fluids used can be drilling fluid or any other fluid known and used in the art for removing gelled drilling fluid and filter cake from the walls of a well bore. In addition, the drill string can be removed from the well and the drill bit removed and replaced with a fluid diverter on the end of the drill string. The string can then be lowered back into the well and the fluid can be displaced through the drill string and the diverter which has holes defined therethrough. Finally, mechanical scrapers have been attached to the casing so that as the casing is run into the well bore it physically contacts and breaks up some of the drilling fluid and filter cake.
While the above described methods and other prior art techniques have achieved varying degrees of success, there is a need for improved methods and apparatus for removing filter cakes and gelled drilling fluids from the walls of a well bore prior to removing the drill string from the well bore and without the need for subsequent trips into the hole with the drill string.
There is also a need for methods and apparatus which can remove gelled fluid, filter cake and other debris from the riser pipe and subsea assemblies used in the drilling of offshore wells without the need for special trips in and out of the offshore well. The riser pipe, as is well known in art, is a flexible pipe which connects an offshore rig to the subsea assembly. The subsea assembly, which is located on the sea floor over the wellbore, typically includes equipment such as, but not limited to, the well head and blowout preventers. The drill string utilized to drill the well passes through the riser pipe and subsea assembly so that during drilling operations, drilling fluid will circulate through the riser pipe and subsea assembly, and gelled mud, filter cake and other debris will build up thereon. The gelled mud and filter cake must be removed periodically so that the equipment in the subsea assembly is not damaged and so that the operation of such equipment is not impaired. Likewise, gelled mud and filter cake must be removed from the internal diameter of the riser pipe so that drilling operations are not impaired. If the buildup is not removed periodically, it can become so great that chunks of filter cake and other debris will break free. Such chunks can damage the subsea assembly and impair drilling operations.
To date, the primary method employed to remove the build up of filter cake and gelled mud from the subsea assemblies and riser pipe is simply to remove the drill string from the well, remove the drill bit and attach a jetting tool on the end of the drill string as opposed to the drill bit. The jetting tool, as is well known in the art, has a plurality of ports defined therethrough, with the ports having nozzles received therein. Once the drill bit has been replaced with the jetting tool, the drill string is lowered through the riser pipe and subsea assembly and then removed therefrom as fluid is circulated through the drill string. The fluid circulated through the strings will exit the tool at a high velocity through nozzles thereby cleaning the subsea assembly and removing gelled mud, filter cake and any other debris from the inside of the subsea assembly and riser pipe. Although the above described method will adequately clean the subsea assembly and remove the gelled mud and filter cake, there is a need for method and apparatus which will clean the subsea assembly and remove debris from the inside of the riser pipe without the need for special trips in and out of the well for the sole purpose of cleaning the subsea assembly and riser pipe.